The present invention relates to microwave components with an at least partially enclosed cavity which are suitable for mass production and which satisfy high quality requirements. Examples of such microwave components are microwave filters, waveguides and horn antennas. The invention further relates to a method of manufacturing such components.
The manufacture of products of the above-mentioned kind has up to now been very complicated and expensive. Today the manufacture is primarily performed by working aluminium, inter alia by high-speed milling and subsequent surface finishing, such as silver-plating, coating, etc. As a result, it is time-consuming to manufacture each component and a great number of manual operations are necessary. Furthermore, it is difficult to obtain the desired dimensional tolerances and quality of the product by this manufacturing process. Thus, as a rule these products need considerable after-treatment.
To solve these problems, the filter casings have, for instance, been provided with trimming means, which allow trimming of the filters after final assembly. However, this makes the filters even more complicated and expensive to manufacture. Moreover, this makes it necessary to test and trim each filter separately by a specialist.
The manufacturing process also significantly limits the possibility of manufacturing certain component parts. High-speed milling allows milling of simple geometric designs only, which makes it necessary to manufacture complicated geometric designs in several pieces, which are subsequently assembled into one functional unit. However, such assembly of several subcomponents into a microwave component almost inevitably leads to a lower degree of dimensional accuracy in the final product, which results in an even greater need for trimming, for instance, of filters after assembly. To arrange trimming means on the filters is time-consuming and considerably increases the costs.
The use of trimming means, such as trimming screws, and the assembly of products from several including parts also constitute a risk of electric disorders, so called passive intermodulation (PIM). In some applications, this can be disastrous.
The making of the structural or supporting parts of aluminium also limits the thermal dimensional stability and the weight.
As an alternative, it has been proposed in JP 61 079 303 to manufacture waveguides on fusible cores. Around this core, silver and copper are plated, and a carbon fibre fabric is subsequently wound around the core until a thickness of about 2 mm. During the winding, the fabric is impregnated with epoxy resin, and the wound support structure is subsequently cured by supplying heat and pressure, after which the core is melted out. The resulting waveguide consists of a composite structure having continuous carbon fibres with an inner layer of copper and silver.
However, also this manufacturing method suffers from a number of drawbacks. The method is expensive and complicated and requires a great number of manual operations. Thus the method is not suitable for mass production, and the manufacturing time for each component is long and the costs are high.
In addition, the technique is not applicable to the manufacture of filter casings, since it is not possible to wind the carbon fibre fabric in the narrow, downwardly projecting, often circular cavities in the filter casings, or corrugations in horn antennas.
Furthermore, in the prior-art wound carbon fibre waveguide the copper layer cannot affect the rigidity and the thermal stability of the component. In this case, the higher e-module of the carbon fibre structure completely dominates the copper layer, and at temperature changes, which frequently occur in microwave components, this may cause micro-cracking problems in the metal layer. Other problems that may arise are reduced adherence of the composite to the metal and galvanic corrosion due to humidity entering the waveguide through the cracks. The presence of micro-cracks in microwave components, and especially microwave filters, immediately results in reduced electric properties.
It is also a problem with prior-art microwave components that the sensitive electric layer, which internally faces the cavity, often gets damaged either during the manufacturing process or during the use of the component due to different types of environmental influence. This is very serious, since it considerably changes and deteriorates the qualities of the component and usually makes it necessary to replace and discard the component.
Consequently, there is a need for microwave components which can be manufactured at a lower cost and in a more efficient manner, in particular on a large scale, and which also provide better products, have a greater resistance against environmental influence, improved dimensional accuracy, improved thermal dimensional stability, fewer including parts to be integrated and improved electric properties.
Thus, the object of the present invention is to provide microwave components with cavities, which wholly or at least partly obviate the above-mentioned problems. The invention also provides a method of manufacturing such microwave components.
This object is achieved by means of a microwave component and a method according to the appended claims.
The invention relates to microwave components with an at least partially enclosed cavity, comprising an outer support structure and an electric layer, which is preferably made of silver and which is arranged on the inside of the support structure. The microwave components according to the invention are distinguished in that they further comprise a first inner protective layer of gold (D), said protective layer being arranged on the electric layer (C) and facing the cavity.
The protective layer is preferably a chemically precipitated gold layer. By arranging such a protective layer, the sensitive electric layer is protected against environmental influence and damage, at the same time as the electric function is not affected to any substantial degree. Unlike prior-art methods of protecting silver surfaces for electric use in microwave components, a gold layer arranged directly on the silver surface has the advantage that it can be made thin, yet completely tight, and it also provides a lasting protection against the environment. In contrast to galvanically applied gold, a chemically applied gold layer provides completely tight layers in the small thicknesses that are electrically acceptable in these connections.
The structure of the electric layer is of great importance. Silver offers by far the best electric properties compared with other conducting materials. The electric properties have a great influence on the performance of microwave components, The application of silver by pulse-plating additionally improves the evenness and tightness of the layer. Pulse-plated silver also permits satisfactory macro spreading, thus allowing plating in narrow spaces, which is not possible by conventional direct-current plating. This is crucial as the cavities almost exclusively have partial surfaces and edges that are located at different distances from the power source. The addition of a protecting chemically precipitated gold layer on the silver layer has surprisingly been found to offer many advantages. A chemically precipitated gold layer is considerably tighter than, for instance, galvanically precipitated gold layers. One advantage of chemically precipitated gold on pulse-plated silver is thus that the even and tight silver is protected by a gold layer which is very thin but still tight. The alternative of using a galvanically applied gold layer requires a considerably thicker layer to attain the same tightness, usually more than ten times thicker. Microwaves in a component penetrate into the metal layers and a great disadvantage of galvanically applied layers is that the thicker gold layer reduces the electric properties of the component due to the lower conductivity of the gold. In addition, the inferior electric properties are further deteriorated since the composition of the layer will be uneven as a consequence of the uneven distribution of the field strength. From the point of view of production, galvanically applied layers are also disadvantageous, compared with chemically precipitated gold, due to longer manufacturing time, increased thickness margin owing to unevenly composed layers, higher material costs as well as higher weight.
An alternative way of applying a gold layer is to passivate silver, for instance, with an organic substance. But this is disadvantageous for several reasons. Unlike the precious metal gold, organic substances react with a number of substances which can change the composition of the surface. Organic substances allow diffusion of substances through the layer to a considerably larger extent and thus cannot afford such a complete protection. The organic layer is less resistant to high field strength. The organic layer has less temperature resistance and less resistance to decomposition. When using organic layers, it is more important that the layers be thin as organic layers are not electrically conducting and thus have a detrimental effect on the electric properties, such as conductivity. An organically composed layer does not provide the same mechanical strength as a metal gold layer. As a consequence, there is a considerably increased risk of the layer breaking through in contact surfaces and other surfaces exposed to wear. If this happens, the electric signals can be influenced in an uncontrollable manner by the occurrence of differences in conductivity and insulation in the component.
On the other hand, it has surprisingly been found that the arrangement of a protective layer of chemically precipitated gold provides excellent protection against environmental influence on the electric layer, at the same time as the layer can be made so thin that the electric properties of the component will not be affected to any appreciable extent.
Furthermore, the outer support structure is preferably made of a cast material, such as a castable metal or a ceramic or plastic material, and made in one integral piece. By using a castable material for the manufacture, the dimensional accuracy increases essentially, at the same time as the manufacturing can be performed in a rapid and efficient manner and is thus well suited for mass production of such components. Unlike, for instance, wound carbon fibre fabric, an integral support structure has omnidirectional mechanical and thermal properties. This is a great advantage, especially in case of complicated geometric designs, such as cavities in filter casings and corrugations in horn antennas. In addition, it is usually these geometric designs that have the narrowest tolerances of the components. The provision of a support structure with omnidirectional properties therefore contributes to a great extent to achieving satisfactory repeatability in mass production.
The outer support structure can, as an alternative, be composed of one or more metal layers against the conducting silver layer.
Thanks to the improved properties of the microwave component according to the invention relative to parts which are formed by after-treatment, such as high-speed milling, and which are manufactured by winding or the like, the finished component can be provided without trimming. This means that it is possible to guarantee such a quality that extra trimming means, which were formerly necessary in many connections, can be omitted which results in considerable savings. Furthermore, the PIM-levels will be very low and in most cases substantially negligible. Depending on the choice of material, improved dimensional stability under heat, a lower weight of the product, improved environmental resistance and extremely good dimensional accuracy are also obtained.
Thanks to the use of the cast or plated outer support structure, it is also possible to provide geometrically complicated microwave components, such as integrated filter casings, waveguide systems and similar put-together products made in one piece, which facilitates assembly and reduces the risk of electric loss.
The composition structure according to the invention is in particular suitable for microwave components with cavities for telecommunication, comprising a partially enclosed cavity and electric connections arranged on at least one side of said cavity. The tolerance requirements for this type of component are very critical, and therefore there is a great need of an improved product which reduces the need of after-treatment and trimming. Due to the fact that the outer support structure is made in one integral piece, it is also possible to manufacture the entire microwave component, including the inner walls and the like and electric connections for the coupling to the rest of the waveguide system, in one piece. Consequently, it is possible to obtain high functionality within a small volume.
For essentially the same reasons, the inventive structure is further suitable for waveguides for microwaves, which waveguides comprise a cavity and electric connections arranged on at least one side of said cavity. The invention is particularly suitable for waveguides in which the cavity is bent in at least one plane and preferably in a plurality of planes. Such complicated geometric designs are substantially impossible to produce in one piece by present-day techniques. It is also possible to provide waveguides in which the cavity is twisted by means of the inventive structure.
The outer support structure of the microwave components according to the invention preferably has such dimensional tolerance and thermal stability at the inner surface that the electric requirements can be fulfilled without trimming. Thus the need of after-adjustment and trimming during assembly is avoided as well as the need of arranging trimming means on the component.
It is also possible to choose a material for the outer support structure that is at least partially flexible and which allows at least some degree of twisting or bending of the component. As a result, some degree of flexibility can be imparted to the cavities of microwave components, and one type of component can be used in a great number of applications. This increases the usability of each product and improves the possibilities of mass production in greater series.
Furthermore, for many purposes the outer support structure preferably comprises zinc, tin or alloys of these materials, since all these materials are castable and have very good properties as regards thermal stability.
On the other hand, for other purposes the outer support structure preferably comprises epoxy plastic material, which is further preferably filled with reinforcing particles of harder material, such as micro-carboys or homogeneous micro-spheres, which particles preferably have a size in the range of 10-350 xcexcm. The particles, which can also be used as filling in castable metals, increase the rigidity and the thermal stability of the material.
As concerns the dimensions, the outer support structure preferably has a thickness that is less than 5 mm and the electric layer a thickness that is less than 10 xcexcm.
The inventive microwave component preferably comprises an inner support structure made, for instance, of copper, said support structure being arranged between the outer support structure and the electric layer and adapted to impart improved thermal stability and/or mechanical strength to the component in interaction with the outer support structure. The use of two support structures, one outer that is cast or plated in one or more layers, and one inner that is for instance plated, provides an often necessary possibility of trimming the mechanical and thermal properties of the components by the choice of material combinations and layer thicknesses of the structures. The thus-obtained interaction between the outer and the inner support structure is particularly important when manufacturing microwave components with cavities in one piece, which components lack after-trimming means. The tolerance requirements as to the dimensions in this application are usually extremely narrow and often less than 10 xcexcm. The inner support structure advantageously has a thickness of between 5 and 200 xcexcm. The inner support structure, which preferably consists of copper, affects the rigidity and thermal stability of the component and increases the adhesion of the inner surface. Unlike prior-art solutions, none of the support structures will in this case totally dominate the other, which guarantees an efficient interaction between them. The support structure can be composed of one or more layers.
It is also suitable for the protective layer to be arranged on the electric layer, preferably so as to cover the same completely, and to have such a small thickness, preferably less than 0.5 xcexcm, that the electric properties of the component are not affected to any considerable extent.
In many cases, a protective layer, for instance of chemically precipitated gold, is preferably arranged on the outer layer. It may also be advantageous to arrange a protective layer between the inner and the outer support structure when the outer support structure is not made of metal. In this way, the inner layers are protected against outside environmental influence.
The invention also relates to a corresponding method of manufacturing the microwave components according to that stated above.